A DC arc furnace can have consumable arcing electrodes made of either solid graphite or of the Soderberg type. The furnace hearth which contains the melt has a melt contact electrode electrically contacted by the melt and which for practical reasons is offset in the horizontal direction from the arcing electrode which is conventionally positioned vertically through the furnace roof.
With the above construction the circuit is through the melt contact electrode, the melt, the arc, and the arcing electrode when the furnace is powered with DC. The offset position of the melt contact electrode, also called the hearth electrode, causes the arc to be directed obliquely with respect to the arcing electrode. This causes the arc flare to radiate most intensely against the furnace side wall and the furnace roof portion which are in the angular direction of the arc.
Such a DC arc furnace may use more than one arcing electrode and more than one hearth electrode. The prior art has suggested that a number of hearth electrodes be used symmetrically arranged around the arcing electrode for the purpose of balancing the electromotive forces involved to thereby keep the arc in alignment with the arcing electrode. Due to the service conditions involved a hearth electrode is expensive to construct in a furnace and it has other objectionable features. Therefore, it is desirable to use only a single hearth electrode. Varying the number of arcing electrodes is not relevant with respect to the problem under discussion.
For a smooth arc and long electrode arcing life it is usual to power such a DC arc furnace so that the melt contact or hearth electrode operates as an anode and the arcing electrode or electrodes operate cathodically.
So that only one hearth electrode can be used while the arc is held in axial alignment with the arcing electrode, the Stenkvist U.S. Pat. No. 4,016,355, dated Apr. 5, 1977, assigned to the assignee of the present invention, discloses the use of two magnetic cores forming a cross and with vertically extending pole pieces each supplied with an electric solenoid individually supplied with DC, this electromagnetic construction being positioned beneath the bottom of the furnace. The furnace bottom is made of non-magnetic materials and this electromagnetic assembly or system is positioned below the furnace bottom with its upstanding pole pieces symmetrically arranged around the vertical axis of the arcing electrode.
The disclosure of the above Stenkvist patent is by reference hereby incorporated into the present disclosure.
With the above patented arrangement the power can be supplied to the four solenoids so as to control the arc direction. The purpose is, of course, to keep the arc in vertical alignment with the arcing electrode so that the arc flare is uniformly distributed around the furnace side wall and the peripheral portion of the furnace roof to thereby distribute the erosion caused by the arc flare, uniformly throughout the furnace.
Although reasonably successful in achieving its intended purpose, the above patented arrangement has involved the problem that in practical furnace applications it has proven difficult to construct the crossed cores, pole pieces and solenoids and to power the solenoids with such precise accuracy as to make this patented concept perfectly satisfactory. Magnetic field dissemetrics around the arc are possible. Too frequently the arc when once electromagnetically aligned with the arcing electrode, becomes unstable so that the arc direction changes and assumes more or less the obliquity which produces the arc flare directed locally and intensely in the arc's angular direction against the side wall and adjacent portion of the furnace roof.
In other words, holding the arc accurately aligned with the arcing electrode of a DC arcing furnace, without using a multiplicity of melt contact or hearth electrodes, has not heretofore been solved to the fullest possible degree.